Theories of Theories of Human Human

Dispersal. Dispersal.

The Replacement Theory (or The Replacement Theory (or Mitochondrial Eve and Y Mitochondrial Eve and Y

chromosome Adam )chromosome Adam )

vs vs

The Multiregional Theory. The Multiregional Theory.

ExamininExamining the g the

evidenceevidence

The Replacement The Replacement Theory (Out of Theory (Out of

AfricaAfrica

Hypothesis)Hypothesis)

This section is the part This section is the part that both theories have that both theories have in common, but in the in common, but in the

Multiregional Hypothesis Multiregional Hypothesis gene flow continues, gene flow continues,

while in the Replacement while in the Replacement Theory there is little or Theory there is little or

no gene flow. no gene flow.

African Origin

European Homo erectus

European Archaic Asian ArchaicAfrican Archaic

EXTINCT EXTINCT

Asian ModernAfrican ModernEuropean Modern

Asian Homo erectus

African Homo erectus

The Out of Africa model The Out of Africa model contains the following contains the following

components:components:• After Homo erectus migrated out of Africa the After Homo erectus migrated out of Africa the

different populations became reproductively different populations became reproductively isolated, evolving independently, and in some isolated, evolving independently, and in some cases like the Neanderthals, into separate cases like the Neanderthals, into separate species. LITTLE OR NO GENE FLOW.species. LITTLE OR NO GENE FLOW.

• Homo sapiens arose in one place, probably Homo sapiens arose in one place, probably Africa (geographically this includes the Middle Africa (geographically this includes the Middle East) East)

• Homo sapiens ultimately migrated out of Homo sapiens ultimately migrated out of Africa and replaced all other human Africa and replaced all other human populations, without interbreeding. populations, without interbreeding.

• Modern human variation is a relatively recent Modern human variation is a relatively recent phenomenon. phenomenon.

The Replacement Theory is The Replacement Theory is most strongly supported by most strongly supported by

genetic evidence genetic evidence • Allan Wilson Allan Wilson

contributed much contributed much research on the research on the mitochondrial “Eve” mitochondrial “Eve” theory – tracing theory – tracing human women of human women of today back to a today back to a common ancestor common ancestor (really a group of (really a group of women) who lived in women) who lived in Africa about 150,000 Africa about 150,000 years ago. years ago.

Mitochondrial DNAMitochondrial DNA

•DNA is present inside the DNA is present inside the nucleus of every cell of our nucleus of every cell of our body but it is the DNA of the body but it is the DNA of the cell’s mitochondria that has cell’s mitochondria that has been most commonly used been most commonly used to construct evolutionary to construct evolutionary trees. trees.

• Mitochondria have their own genome of Mitochondria have their own genome of about 16,500 bp that exists outside of about 16,500 bp that exists outside of the cell nucleus. Each contains 13 the cell nucleus. Each contains 13 protein coding genes, 22 tRNAs and 2 r protein coding genes, 22 tRNAs and 2 r RNAs RNAs

• Mitochondria are present in large Mitochondria are present in large numbers so fewer samples needed.numbers so fewer samples needed.

• Mitochondrial DNA has a higher rate of Mitochondrial DNA has a higher rate of substitution mutations than nuclear DNA substitution mutations than nuclear DNA making it easier to resolve differences making it easier to resolve differences between closely related individuals. between closely related individuals.

• Mitochondrial DNA is inherited only Mitochondrial DNA is inherited only from the mother, which allows tracing from the mother, which allows tracing of a direct genetic line. (Mitochondria of a direct genetic line. (Mitochondria are found in the midpiece of the are found in the midpiece of the fertilising sperm, which remains fertilising sperm, which remains outside the fertilised egg) outside the fertilised egg)

• Mitochondrial DNA does not recombine Mitochondrial DNA does not recombine – the process of recombination in – the process of recombination in nuclearnuclear DNA mixes sections of DNA DNA mixes sections of DNA from the mother and the father from the mother and the father creating a garbled genetic history. creating a garbled genetic history.

• More recently, research More recently, research on Y chromosome on Y chromosome markers has supported markers has supported the replacement the replacement hypothesis and traced hypothesis and traced origins of today’s human origins of today’s human males to a common males to a common ancestor ( or group of ancestor ( or group of men) approximately 60 – men) approximately 60 – 90,000 years ago) 90,000 years ago)

• Both of these groups are Both of these groups are placed in Africa placed in Africa

The Y Chromosome has The Y Chromosome has certain Unique Featurescertain Unique Features

• The presence of the Y Chromosome causes The presence of the Y Chromosome causes maleness. This little chromosome – about 2% of maleness. This little chromosome – about 2% of a father’s genetic contribution to his sons – a father’s genetic contribution to his sons – programs the early embryo to develop into a programs the early embryo to develop into a male.male.

• It is transmitted from fathers only to their sons.It is transmitted from fathers only to their sons.• Most of the Y chromosome is inherited as an Most of the Y chromosome is inherited as an

integral unit passed without alteration from integral unit passed without alteration from father to sons and to their sons. It is unaffected father to sons and to their sons. It is unaffected by recombination and is the only nuclear by recombination and is the only nuclear chromosome that escapes the continual chromosome that escapes the continual reshuffling of parental genes during the process reshuffling of parental genes during the process of sex cell production. of sex cell production.

• The Y chromosome is estimated to carry The Y chromosome is estimated to carry about 78 working genes with 97% of its about 78 working genes with 97% of its length “junk” DNAlength “junk” DNA

• Junk DNA is not subject to change by Junk DNA is not subject to change by natural selection so variation is only due to natural selection so variation is only due to mutation.mutation.

• Thus, Y chromosomes pass down from Thus, Y chromosomes pass down from father to son largely unchanged except by father to son largely unchanged except by the gradual accumulation of mutations the gradual accumulation of mutations (polymorphisms) (polymorphisms)

• Certain polymorphisms are selected to act Certain polymorphisms are selected to act as genetic markers and the changes in as genetic markers and the changes in them tracked over generations. them tracked over generations.

What’s on the Y What’s on the Y Chromosome?Chromosome?

Multi – regional Multi – regional (Regional (Regional

Continuity) Theory.Continuity) Theory.

Multi-regional Theory – Multi-regional Theory – contains the following contains the following

components:components:• Some level of gene flow between Some level of gene flow between

geographically separated populations geographically separated populations prevented speciation, after the dispersal.prevented speciation, after the dispersal.

• All living species derive from the species All living species derive from the species Homo erectus Homo erectus that left Africa nearly 2 million that left Africa nearly 2 million years ago. years ago.

• Natural selection in regional populations, ever Natural selection in regional populations, ever since their original dispersal, is responsible for since their original dispersal, is responsible for the regional variants (sometimes called races ) the regional variants (sometimes called races ) we see today. we see today.

• The emergence of Homo sapiens was not The emergence of Homo sapiens was not restricted to any one area but was a restricted to any one area but was a phenomenon that occurred throughout the phenomenon that occurred throughout the entire geographic range where humans lived. entire geographic range where humans lived.

African Origin

• The Multiregional Hypothesis is The Multiregional Hypothesis is more supported by fossil evidencemore supported by fossil evidence

• The current best explanation for the The current best explanation for the biginning of modern humans is the Out of biginning of modern humans is the Out of Africa Model that proposes a single African Africa Model that proposes a single African origin for origin for Homo sapiens. Homo sapiens. The major The major neurological and cultural innovations that neurological and cultural innovations that characterized the appearance of fully characterized the appearance of fully modern humans has proven to be modern humans has proven to be remarkably successful, culminating in our remarkably successful, culminating in our dominance of the planet at the expense of dominance of the planet at the expense of all earlier hominid populations. all earlier hominid populations.

• This theory is best supported by the genetic This theory is best supported by the genetic evidence while the multiregional theory is evidence while the multiregional theory is best supported by the fossil evidence. best supported by the fossil evidence.

• But the genetic evidence has a But the genetic evidence has a crucial advantage in determining the crucial advantage in determining the structure of family trees: structure of family trees: living genes living genes must have ancestors, whereas dead must have ancestors, whereas dead fossils may not have descendants. fossils may not have descendants.

• Molecular biologists know the genes Molecular biologists know the genes that they are examining must have that they are examining must have been passed through lineages that been passed through lineages that survived to the present; survived to the present; paleontologists cannot be sure that paleontologists cannot be sure that the fossils they examine do not lead the fossils they examine do not lead down an evolutionary "blind alley". down an evolutionary "blind alley".

Questions relating to the article by Questions relating to the article by Donald Johanson “Origins of Donald Johanson “Origins of

Humans: Multiregional or Out of Humans: Multiregional or Out of Africa?”Africa?”

1.1. What are the scientific names of all What are the scientific names of all the early ancestors of humans the early ancestors of humans including the name for modern including the name for modern humans? humans?

2.2. Define the following terms: gene flow; Define the following terms: gene flow; speciation; taxonomic diversity; speciation; taxonomic diversity; genetic drift; occipital region of the genetic drift; occipital region of the skull; molars; cranial vault; Paleolithic skull; molars; cranial vault; Paleolithic era; mtDNA; Neanderthals. era; mtDNA; Neanderthals.

3.3. How many years ago did all modern How many years ago did all modern humans become anatomically and humans become anatomically and behaviourally similar.?behaviourally similar.?

4.4. What are the three types of evidence What are the three types of evidence that are used to debate the 2 models of that are used to debate the 2 models of human origins?human origins?

5.5. What was the “significant innovation” What was the “significant innovation” that occurred in the Upper Paleolithic that occurred in the Upper Paleolithic of Eurasia? of Eurasia?

6.6. When did Neanderthals disappear? When did Neanderthals disappear? 7.7. Describe the physical characteristics of Describe the physical characteristics of

each of the early ancestors of humans each of the early ancestors of humans and compare those characteristics to and compare those characteristics to those of modern humans, as described those of modern humans, as described in the article. in the article.

8.8. Summarize Klein’s idea of what Summarize Klein’s idea of what brought about the emergence of brought about the emergence of behaviourally modern humans.behaviourally modern humans.

10.10.What countries are represented in What countries are represented in the evidence on the origins of the evidence on the origins of modern humans? modern humans?

11.11.How are Peking Man and Java Man How are Peking Man and Java Man different from modern humans? different from modern humans?

12.12.What is interbreeding and what What is interbreeding and what role did it play in the development role did it play in the development of modern humans, according to of modern humans, according to the Out of Africa model?the Out of Africa model?